When chlorine is passed through a sample of water, the pH of the water sample would be

Analyzing the question

We are asked what happens to the pH of water when chlorine gas is dissolved in it.

Concept

Chlorine reacts with water in a reversible reaction to form two acids:

Cl₂ + H₂O ⇌ HCl + HOCl

• HCl (hydrochloric acid) is a strong acid and ionizes completely.
• HOCl (hypochlorous acid) is a weak acid.

Both produce H⁺ ions, so the solution becomes acidic.

Solution

1. Pure water has a pH of 7 (neutral).
2. Chlorine dissolves and reacts with water to form HCl and HOCl.
3. HCl → H⁺ + Cl⁻ (complete ionization), which greatly increases the H⁺ concentration.
4. HOCl ⇌ H⁺ + OCl⁻ (partial ionization), which adds a little more H⁺.
5. Since [H⁺] increases above 10⁻⁷ mol/dm³, the pH falls below 7.

This is also why chlorine water turns blue litmus paper red first, and then bleaches it white because of the HOCl.

Checking the options

• A. < 7: correct, because the solution is acidic.
• B. > 7: wrong, because this would be basic.
• C. = 7: wrong, because this would be neutral.
• D. 0: wrong. A pH of exactly 0 would need a very concentrated strong acid, and chlorine water is only mildly acidic (the pH is typically about 2 to 4).

Answer: A (pH < 7)

Exam tip: Remember "Chlorine water = HCl + HOCl, so acidic, then bleaching." Non-metal oxides and halogens dissolved in water generally give acidic solutions, so look for pH < 7.